Vacuum plasma processors are used to deposit materials on and etch materials from workpieces that are typically semiconductor, dielectric and metal substrates. A gas is introduced into a vacuum plasma processing chamber where the workpiece is located. The chamber pressure is typically in the range of 0.1 to 1000 torr. The gas is ignited in an excitation region into an RF plasma in response to an RF electric or electromagnetic field. The RF field is provided by a reactive impedance element, usually either an electrode array or a coil which couples both magnetic and electric RF fields to the gas. The reactive impedance element is connected to a first RF source having a first relatively high RF frequency and sufficient power such that the gas is ignited into the plasma. Connections between the first RF source and reactive impedance element are usually by way of a relatively long cable, connected directly to the first RF source. A first resonant matching network connected between the cable and reactive impedance element usually includes a pair of variable reactances adjusted to match the impedance of the first RF source to the load it is driving.
Typically, the gas is introduced into the chamber through the top of the chamber and is withdrawn from the bottom of the chamber. It is common for an electrode at the top of the chamber to be associated with a series of gas baffles and openings into the excitation region to provide a shower head effect for the gas flowing into the excitation region.
The workpiece is usually mounted on an electrode at the bottom of the excitation region. In some chambers, the electrode carrying the workpiece (frequently referred to as the bottom electrode) is the reactive impedance element supplied with the first RF frequency and another electrode spaced from the electrode carrying the workpiece (frequently referred to as the top electrode) is connected to a reference potential, typically ground. Processors with such configurations are frequently referred to as diodes because the chamber includes one powered electrode, a grounded wall structure and a grounded electrode.
In other processors, the top and bottom electrodes of the chamber are respectively powered by separate relatively high and low RF frequencies and the chamber wall structure is grounded. The relatively high frequency, which is in excess of 10 MHz and is typically 27 MHz, controls the density of the plasma, while the relatively low frequency, which is less than 10 MHz and is typically 2 MHz, controls the energy of ions in the plasma. Independent control of plasma density and ion energy is provided by independently controlling electric parameters, such as power, voltage and/or current, of the first and second RF sources. Chambers having this configuration, frequently referred to as triodes, also provide control for a DC bias voltage established on the bottom electrode. The DC bias voltage control results from an interaction between the plasma close to the top and bottom electrodes and RF fields the top and bottom electrodes couple into the plasma in response to the RF power applied to the top and bottom electrodes. By varying the relative values of parameters, e.g., current and/or voltage, or power, applied to the top and bottom electrodes, the bottom electrode DC bias voltage changes in a controllable monotonic manner so that as the high frequency power increases relative to the low frequency power, the magnitude of the DC bias voltage, which is negative, increases.
It is known for chambers having the foregoing diode and triode configurations to include a structure for confining the plasma to a confinement region including a volume between the electrodes. The confinement structure prevents the plasma from being incident on the chamber grounded metal wall structure, to thereby prevent substantial damage to the chamber wall structure and increase control of the plasma properties; see, e.g., Lenz et al., U.S. Pat. No. 5,534,751.
In still other chambers, wherein the top electrode and the chamber metal wall are connected to ground (a reference potential), relatively high and low RF frequency power is simultaneously supplied to the bottom electrode. The vacuum chamber grounded metal wall structure of such diode configurations usually defines the exterior of a region where the excited plasma subsists. In particular, Lenz, U.S. Pat. No. 5,998,932, discloses simultaneously supplying 2 MHz and 27 MHz to the bottom electrode of a chamber with a confinement structure and grounded top electrode and metal wall. Kuthi et al., U.S. Pat. No. 6,106,663, also discloses simultaneously supplying 2 MHz and 27 MHz to the bottom electrode of a chamber with a grounded top electrode.
Prior art processors with chambers having a configuration wherein high and low frequencies simultaneously drive the bottom electrode have been constructed in such a manner that the DC bias voltage on the bottom electrode is not controllable by varying the relative values of high and low frequency parameters, e.g., power, supplied to the bottom electrode. It has been found that the DC bias voltage remains relatively constant as the relative amounts of high and low frequency power supplied to the bottom electrode vary. We believe the bias voltage remains relatively constant because the chambers have been constructed so the ion energy (which is supposed to be controlled primarily by the low frequency power) is coupled substantially with the ion density (which is supposed to be controlled primarily by the high frequency power). Consequently, there is coupling of plasma density with the width of a sheath between the edges of the plasma volume and surfaces of the chamber close to the plasma edges. As a result, varying the relative values of a parameter (e.g., power) at the high and low frequencies has not resulted in the desired independent control over plasma ion energy and plasma density. Thus, the prior art diode chambers wherein two different frequencies are simultaneously applied to the bottom electrode have been unable to come close to achieving the degree of bottom electrode bias voltage control that triode dual frequency chambers have attained.